1. Field of the Invention
The present invention relates to a granulating method by forming droplets from a liquid material using a continuous ink-jet method and drying the droplets, a granulated particle(s) and a granulating device.
2. Description of the Related Art
As one of granulating methods for producing particles such as powder, granule and so on, is known that a liquid containing a raw material is made into a fine droplet and solidify this droplet by drying or the like. And a technology for producing a high-quality granulated powder has been examined, e.g. a method to form a droplet or to alter the droplet to a particle.
As a granulating method by spray drying, Patent Document 1 (Japanese Patent Laid-Open No. 2001-070779) discloses a technology that a fine particle of a substance to be granulated is carried into a chamber and this particle is accompanied in a dry motion gas flow and given spray of a granulating droplet for granulation. And Patent Document 2 (Japanese Patent Laid-Open No. 2004-082005) discloses a technology that a droplet is generated from a solid-particle dispersed solution continuously flowing through a spray drying chamber using a surface tension and a centrifugal force of a disk atomizer and granulating it by drying.
Also Patent Document 3 (Japanese Patent Laid-Open No. 2004-027313) discloses a technology that in granulating a material powder to be used in a powder metallurgy product, slurry is discharged from a vibrated porous nozzle to discharge the slurry in a divided state and a granulated powder with controlled shape is obtained by quickly drying this divided slurry. And Patent Document 4 (Japanese Patent Laid-Open No. 08-059352) discloses a technology that slurry is dropped on a smooth dropping face having high repellency and its shape is maintained as a droplet in the spherical shape or its deformed shape by surface tension and dried. Moreover, Patent Document 5 (Japanese Patent Laid-Open No. 2005-137955) discloses a technology of producing a particle using an ink-jet method, drying and baking it.
These technologies to produce granulated powder are used in various fields including powder metallurgy products, chemicals, cosmetics, foods and so on. And the particle material which has a required quality, particle diameter, uniformity of particle diameters and the like, are produced depending on the application.
As a quality index of a granulated particle in granulation of a powder material, an average particle diameter and a particle size distribution are used. And it is desired a high-quality granulated particle(s) which has been controlled the average particle diameter and particle size distribution with a high accuracy. If the uniformity of the particle diameter of the granulated powder is low and the particle size distribution is broad, the following problems matter. For example, when processing a material containing a particle using a tank or a hopper, so-called hanging phenomenon or aggregation might occur, and this deteriorates workability. Also, only coarse particle powders which is good liquidity flows into a production line at processing, while granulated powders with a small particle diameter which is poor liquidity remains in a tank or the like, and as a result, the product quality might be uneven. Particularly, in a ferrite, dielectric material, battery material or the like which needs baking or sintering after filling a die, the process accompanied by volume change a filling amount is fluctuated by a material powders with unevenness quality. It causes distortion, a crack, a hole and unevenness characteristic values or the like in a product after baking or sintering. And the above related arts of Patent Documents 1 to 5 have the following problems.
When a granular material is produced by the granulating method of Patent Document 1, since the droplet is generated only by surface tension of a continuously flowing granulating liquid, there is a tendency that the particle size distribution broader. Also, since the particle diameter is controlled by the nozzle diameter and a supply amount of the granulating liquid, particle-diameter control is possible but the particle size distribution might become different depending on the condition. And the broader particle size distribution of actual products against a desired particle size distribution is a problem. Moreover, the granulated powder might be withdrawn into a bag filter or the like, which is a problem in yield.
With Patent Document 2, since the continuously flowing solid-particle dispersed solution is made into droplets using surface tension and a centrifugal force of the disk, the particle size distribution becomes broader. And since the particle size distribution is controlled by a rotation speed of the disk and a supply amount of slurry, the control is difficult and combined with broader particle size distribution, it constitutes a problem in yield. Also, since granulated materials with a relatively small particle diameter are withdrawn into a bag filter and the like at drying, the yield is further lowered.
With the granulating method and the granulating device of Patent Document 3, since the diameter of the porous nozzle is not necessarily uniform, the particle size diameter easily becomes broader. Moreover, since droplets are generated by vibrating the porous nozzle to divide the slurry, even if the nozzle diameter is uniform, the holes are closely adjoined to each other and moreover, the droplets are generated substantially at the same timing. Thus, droplets coming from the porous nozzle adhere to each other and generation of deformed particles or the like can not be avoided, which causes a problem in the yield of single dispersed particles. Also, in order to dry the droplets ejected from the porous nozzle instantaneously, a discharge port of the porous nozzle should be located adjacent to a mechanism for drying the droplets. Therefore, the discharge port of the porous nozzle is exposed in a dry state, and especially when it is exposed in a high-temperature dry state, the porous nozzle part can be deteriorated. Moreover, since the porous nozzle itself is constructed to be vibrated so as to generate droplets, there is a possibility of physical and dynamic deterioration such as damage or abrasion of the porous nozzle due to vibration in use, and the deterioration of the porous nozzle during the use is a problem.
With Patent Document 4, since particles are obtained by having generated droplets rolling on a heating surface for drying, deformation of the granulated particles is a problem. The dropping surface used in the granulating method has a possibility that water repellency of the heating surface is lowered and stable spherical particles can not be obtained. Moreover, when a large quantity of particles are to be granulated, there is a possibility that a heat amount of the heated dropping surface does not satisfy a heat amount required for granulation, which is a problem in mass production.
In Patent Document 5, emulsion is used as ink, and a piezo method, a thermal ink-jet method, a bubble-jet (registered trademark) method or the like is used as an ink-jet method. Since the thermal ink-jet method and the bubble-jet (registered trademark) method have the emulsion injected with energy of air bubbles at evaporation of a liquid by heating, a high-temperature heat is applied to the emulsion (ink) and that affects composition of a liquid material and restricts types of granulated particles which can be produced. Also, the surface temperature of a heater used for heating to a temperature to generate air bubbles becomes high, a load is given to a heater, ink, an ink head and the like, and deterioration of those parts in use is a problem. On the other hand, with the piezo method, since the emulsion is discharged by deformation of a piezo element to which a voltage is applied and new emulsion is supplied to the nozzle part, if the viscosity of the emulsion is high, a pressure required for supply of ink becomes high, while if a drive frequency of the piezo element is lowered in response to that, a production drops and desired droplet formation becomes difficult, which is a problem in yield. Also, in a piezo-type ink-jet device, the liquid material is collected in an ink reservoir in an ink-jet nozzle once without fail, it is going to be more likely that a solid component settles and clogs the nozzle according to the composition of the liquid material, if the flow of the liquid material stops even for a short time. That is, there is a problem that many restrictions on the state, contained components, viscosity and the like of the liquid material, to be a material narrow the range of the composition and particle diameter available for granulation.